1. Technical Field
The inventive concept relates to a non-volatile semiconductor memory device and, more particularly, to a phase-change memory device having multiple diodes.
2. Related Art
Phase-change memory devices, which are one type of non-volatile semiconductor memory devices, include phase-change materials of which resistances are varied depending on temperature. Typically, a chalcogenide (GST) based material, which is comprised of germanium (Ge), antimony (Sb) and tellurium (Te), is used as the phase-change material. The phase-change material is changed between a crystalline state and an amorphous state depending on the temperature to define a reset state corresponding to a logic “1” and a set state corresponding to a logic “0”.
A memory cell of the phase-change memory device may include a variable resistor, which is formed from the phase-change material and connected between a word line and a bit line, and a switching element, which selectively drives the variable resistor.
The word line of the phase-change memory device is formed of a junction region in a semiconductor substrate and a bit line is formed of a distribution line. A diode or a MOS transistor may be used as the switching element.
Herein, it is well-known that the junction type word line has a resistance significantly greater than a distribution type word line. Furthermore, if a plurality of memory cells (that is, the switching elements) are connected to the junction type word line, a word line delay may be significant.
Generally, a word line of a metal distribution type (hereinafter, referred to as a metal word line) which is electrically connected to the junction type word line is further formed on the semiconductor substrate to prevent the word line delay. In this case, the switching element is in electrical contact with the metal word line.
When the diode is used as the switching element and selective epitaxial growth is not formed on the metal word line, the diode is formed of a polysilicon layer. As a shottky junction is formed between the polysilicon diode and the metal word line, the polysilicon diode is called a shottky diode.
However, when an etching process is performed to form the shottky diode, an edge of the shottky diode which is in contact with the metal word line may be damaged by an etching gas thereby resulting in a contact fail.
The contact fail between the shottky diode and the metal word line increases an off current and makes an on current difficult to control.